Continuous attic flooding

ABSTRACT

A method for recovering oil is provided wherein gas is injected into a well and oil is produced from the same well simultaneously by injecting the gas through upper perforations in the well at or below a critical rate and producing a volumetrically equal amount of oil through lower perforations in the same well.

United States Patent 1191 11] 3,770,057 Palmer et a]. Nov. 6, 1973 [5 CONTINUOUS ATTIC FLOODING 3,120,264 2/1964 BarronQ 166/257 3,120,265 2/1964 Allen 166/306 [75] lnventors. Harold A. Palmer, Cecil L. Herren, 3,126.96 3/1964 Craig Jr. at alum 66/306 X both of 3,292,703 12/1966 Weber 166/306 [73] Assigneez Texaco Inc. New York, 3,333,637 8/1967 Prats l66/306 X [22] Flled: 1972 Primary Examiner-Stephen J. Novosad [21] Appl. No.: 225,927 Att0rneyThomas H. Whaley et al.

[52] US. Cl. 166/306 [57] ABSTRACT [51] Int. Cl E2lb 43/16 [58] Field Of Search 166/306 305 R 257 A method Pmwded gas 8 injected into a well and oil is produced from the same well simultaneously by injecting the gas through upper [56] References Cited perforations in the well at or below a critical rate and producing a volumetrically equal amount of oil through UNlTED STATES PATENTS lower perforations in the same well. 2,593,497 4/1952 Spearow 166/306 X 3,120,263 2/1964 Hoyt 166/306 X 4 Claims, 5 Drawing Figures 2 NY I WELL 1 3 T\ f f'i 3 f I 71 -1 7 1 OIL 1 r 1 1 Y 1 3 1 I I I 1 r 7 l 1/ i t A 1 J40 l OIL i L RESERVOIR 1 f 1:9 g :1 r. l

SHEET 10F 4 I 5.02 cm LONG FIG!

SCREEN WELL OIL RESERVOIR FIG. 2

PAIENIEDNM- 5 ms 7 1 SHEET 2 er 4 FLOOD 1 -EXTERNAL GAS DRIVE INJECTION 18 ml/h FROM TOP 100 GOR GAS OIL RAT/O; cf/bbl SA TURA T/ON, PERCENT PORE VOLUME RECOVERY, PERCENT STOCKTANK OIL GAS lNJECT/OMPORE VOLUMES 'PAIENIEDNBY elm 3770.057

, SHEET 30F 4 FLOOD 2 SOLUTION GAS DRIVE W/THDRAWAL 36ml/h 4 60 6 B b \l 2 b g g 50 Lu 9 Q: E) 0 89 Q U) h. 40 8000 50 2 K 6 u 2 0 Lu u E 8 2% 30 g v: u: RECOVERY g 20 E a 5 n:

PRESSURE, PS IA RECOVERY, PERCENT STOCK TANK OIL PATENTEDNUY 6 I973 GAS 3770.057 SHEET l 0F 4 FIG. 5

COUNTER CURRENT D/SPLACEMENTS Qml/h (FLOOD 3) 5ml/h (FLOOD 7) IZmI/h (FLOOD 4) I8ml/h(FLOOD 5) 36ml/h (FLOOD 6) I. CONTINUOUS ATTIC F LOODING BACKGROUND OF THE INVENTION This invention is concerned with recovering oil from a subterranean reservoir by adding energy to the reservoir in the form of gas.

Many oil bearing subterranean reservoirs do not possess enough energy to flow oil by natural pressure mechanisms. Consequently energy must be added to such reservoirs to artificially aid in the production of oil. Some such reservoirs lie at an angle from the hori-.

zontal. If a well is not completed at the very apex of such an inclined reversoir the oil above the highest completed well will normally be lost. This oil is called attic oil. A method called attic flooding has been devised to recover this oil without drilling additional wells. In general it works as follows: Gas is injected into I a well near the apex of the formation for a period'of time. Although this injected gas would naturally tend to gravitate upward in the reservoir, the gas usually forms a bubble below the perforations because of the momentum of downward injection coupled-with the resisting forces from the reservoir rock above. These forces would combine to place the lower most point at which the gas would migrate somewhere below the injection perforations. The injection is ceased and the gas injected into the reservoir is allowed to gravitate updip to the apex of the formation displacing the oil in the apex. When the injected gas has migrated a sufficient distance upward from the injection point the well is allowed to produce oil. A repetition of this injectionproduction cycle will eventually remove the oil from the apex of the reservoir above the highest well. It is apparent however, that this method has its drawbacks. The on-again-off-again injection and production cycle will cause obvious operational difficulties. Also, the required waiting period between the ceasing of injection and the beginning of production provides ample opportunity for human error or impatience to cause a too early resumption of oil production with attendant production of some injected gas -that had not migrated sufficiently far from the perforations.

The method of our inventionsolves these problems by providing a continuous attic flooding procedure which will allow oil production and gas injection in the same well bore simultaneously.

SUMMARY OF THE INVENTION The invention is a method for recoveringoil from subterranean hydrocarbon reservoirs penetrated by a well with perforations communicating with the upper Q k sin (d dg)/[(u0/ 0) ("u/ 0] where Q volume injection rate of gas, ml/sec. G gravitational constant, 4.84 l0 atmos. cm

A cross-sectional area, cm

0 angle of dipof formation.

d d density, oil and gas, g/ml.

u u viscosity, oil and gas, centipoise.

k,,, k, permeability to oil and gas, darcies. and producing a volumetrically equal amount of oil through the lower perforations.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 depicts the method of our invention as carried out in a subterranean oil-bearing reservoir;

FIG. 2 shows a cross-section of the core test apparatus used for performing stable counter current fluid flow experiments;

FIG. 3' is a graph showing production performance curves resulting from an external gas drive from the top and production from the bottom;

"FIG. 4 is a graph showing production performance curves as a result of a solution gas drive with production from the bottom; and

FIG. 5 is a graph showing oil production curves using the counter current displacement method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS l The method of our invention may best be described and understood by reference to the attached FIG. 1. A well is shown penetrating an oil reservoir inclined at an angle 0 from the horizontal. The well has upper perforations, P and lower perforations, P separated by a distance L. The natural gravitational forces will cause gas and oil to migrate upward and downward in the formation respectively as shown by the arrows. The method of our invention requires that the gas be injected into the upper perforations at a maximum rate, Q or less, at which rate all the gas will flow updip from the perforations, P and no gas bubble will form below the perforations, P

The lower perforations P are then used to produce an amount of oil volumetrically equal to the amount of injected gas Q at reservoir conditions. The produced oil and injected gas cannot comingle in the well bore. The separation may be accomplished by various ways known to those skilled in the art including packers and separate tubing strings to name only two.

To determine themaxim'um rate Q for gas injection into any reservoir, consider FIG. 1 and the following calculations:

For the gas 1 2 (QD lI g g) d, GL sin 0 where P P, are pressure in atmosphere Q, volume flow rate for gas cc/sec u, gas viscosity, cp k gas permeability, darcies A, cross sectional area through which gas flows, Cm

d, gas density gm/ml L length between constant pressure lines, Cm 6 angle of dip G gravitational and conversion constant 9.68 X 10" atmos. Cm/gm For the oil P,P (Q,,u L/k A d GL sin where the subscript 0 designates oil, then 5 (Q u Llk A d GL sin 0 (Q u L/k A d GL sin 0 For balance, upward volume flow must be equal to downward flow or Q, Q Q

Dropping the Us and solving for Q gives Q=G sin 0 The area of flow to each phase should be inversely proportional to the phase mobility or il n n/ U/( D/h) also A, A A

where A is total cross sectional area. Combining equations 6 and 7 we obtain o/ o i/ l (8) t/ 4s o/ o Substituting'equations 8 and 9 into 5 we obtain Q=G sin 0 i o/ o o (vi/ t 163A. li /k ICU/1 Mo/k0 Rearranging terms Q k sin 0 (d d g/ fl) 0/ 0)] Q is then the maximum volume injection rate of gas at which stable countercurrent flow of gas and oil will occur. In practice it should be the maximum rate at which gas could be injected to have all gas flow updip.

The rate of gas injection may of course be any amount up to and including the calculated rate Q. The preferred rate in most cases would be maximum Q calculated since this would reduce the time for recovery of oil. Any lesser rate, however, is necessitated by circumstances is acceptable. The rate of oil withdrawal must of course be substantially volumetrically equal to the rate of gas injection at reservoir conditions.

The invention is applicable to any reservoir with a dip angle high enough to allow injected gas to migrate at a rate sufficient for Q to be at an acceptable level. Note as the angle of dip, 6 declines the maximum rate of gas injection Q also declines. There is no theoretical lower limit to the angle of dip so long as it has a finite value. However, any angle less than 10 is not likely to provide high enough value for Q to be acceptable. The distance between the injection and production perforations in the well is a function of rate and mobility even more than reservoir thickness. However, as a general proposition the preferred location of the injection perforations should be as near the top of the reservoir bed as possible and production perforations as near to the bottom as possible. Obviously, if the reservoir were too thin, any reasonable rates of injection and production would cause direct channeling from one set of perforations to the other. As a matter of practicality the process is not likely to succeed in fields where the reservoir is too thin to allow the perforations to be more than 20 feet apart. Therefore, it is preferred that this invention be limited to reservoirs where the injection and production perforations may be spaced at least 20 feet apart.

EXPERIMENTAL A test was devised to validate the equation for stable counter current flow derived above.

where Q volume injection rate of gas, ml/sec.

G gravitational constant, 4.84 X 10" atmos. cm 2 A cross sectional area, cm.

0 angle of dip formation.

d,,, d, density, oil and gas, g/ml.

u u viscosity, oil and gas, centipoise.

k,,, k, permeability to oil and gas, darcies.

A core was mounted as shown in FIG. 2. The properties of the core are given in Table l. Properties of the fluids are given in Table II. All experiments were performed at the bubble point of the original mixture, except for Flood 2 described below.

Flood 1 (FIG. 3) was an external gas drive with methane injection at 18 ml/hour at the top and production from the bottom. Flood 2 (FIG. 4) was a solution gas drive with production from the bottom at 36 ml/hour. The remaining five floods were counter current floods. Methane was injected into the side tap at various rates while production was taken from the bottom. Production histories of these floods are shown in FIG. 5. The external gas drive and all the counter current displacements had ultimate recoveries of about 36 percent. The solution gas experiment had a recovery of 24 percent.

A stable injection rate of 10.9 ml/hour was calculated from equation (1). Absolute air permeability was used for both oil and gas effective permeability in this calculation. The two curves in the region of stable counter current flow below 10.9 ml/hour are practically identical. The curves at higher rates show that increasingly greater quantities of gas are required to attain any particular recovery. This constitutes a verification of the equation.

1. A method for recovering oil for subterranean hydrocarbon reservoirs penetrated by a well with perforations communicating with the upper portion of the reservoir and additional perforations communicating with a lower portion of the reservoir, the perforations being separated so that fluids entering or leaving the well through each set of perforations do not commingle in the well bore comprising injecting gas into the reservoir through the upper perforations at a rate from about zero to a maximum rate as calculated from the equation where Q volume injection rate of gas, ml/sec. G gravitational constant, 4.84 X 10" atmos. cm

A cross sectional area, cm

0 angle of dip formation.

d d, density, oil and gas, g/ml.

u u, viscosity, oil and gas, centipoise.

k,,, k,, permeability to oil and gas, darcies. and producing oil through the lower perforations at a rate substantially volumetrically equal to the rate of gas injection at reservoir conditions.

2. A method as in claim 1 wherein the rate of gas injection is from about 80 percent of Q to a maximum of 3. A method as in claim 1 wherein the reservoir dip angle is from about 15 to 90.

4. A method as in claim 1 wherein the distance between the gas injection perforations and the oil production perforations is at least 20 feet. 

1. A method for recovering oil for subterranean hydrocarbon reservoirs penetrated by a well with perforations communicating with the upper portion of the reservoir and additional perforations communicating with a lower portion of the reservoir, the perforations being separated so that fluids entering or leaving the well through each set of perforations do not commingle in the well bore comprising injecting gas into the reservoir through the upper perforations at a rate from about zero to a maximum rate as calculated from the equation Q 1/2 GA sin theta (do-dg)/((uo/ko)(ug/kg)) where Q volume injection rate of gas, ml/sec. G gravitational constant, 4.84 X 10 4 atmos. cm 2/g. A cross sectional area, cm2. theta angle of dip formation. do, dg density, oil and gas, g/ml. uo, ug viscosity, oil and gas, centipoise. ko, kg permeability to oil and gas, darcies. and producing oil through the lower perforations at a rate substantially volumetrically equal to the rate of gas injection at reservoir conditions.
 2. A method as in claim 1 wherein the rate of gas injection is from about 80 percent of Q to a maximum of Q.
 3. A method as in claim 1 wherein the reservoir dip angle is from about 15* to 90*.
 4. A method as in claim 1 wherein the distance between the gas injection perforations and the oil production perforations is at least 20 feet. 